Nayomi - If x is an image, I assume that you mean it is an array. Note that this function is defined explicitly for use on vector input x. If you desire to use it on each row (or column) of an array, you would use an external loop over the rows or columns.
I'm unsure what is meant here. The behavior of filter at the edges is not material, since I explicitly repair those elements along the edges as is appropriate. This is already noted in the help:
% All windowmodes adjust the window width near the ends of
% the series as necessary.
The internal comments also indicate what was done to repair the behavior of filter at the ends.
So I think/thought that this question is already covered, but perhaps the help should be more explicit to reflect what is done. I'll submit a new version that does this.